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Fender M80 bass amp distortion on output

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  • #16
    I know this is an older topic, but as I have just recently repaired a Fender M80 bass amp (mfg Oct 1993) which had very similar, if not the same, problems, I wanted to share what helped in my repair.

    I checked all joints, and all looked ok. The continuity tests with my meter tested ok.
    (This would prove to be a false reading, as detailed later.)

    I cleaned all pots and switches with D100 and F5/F100.

    I replaced all of the electrolytic capacitors. In an amp older than 20 yrs, it never hurts...as long as you're in there, and if you have time, it's a good idea. Using high quality Japanese capacitors is essential.

    Next, I used a scope on the various test points. In doing this, I isolated the distortion problem/wave down to the power amp stage.

    There are 2 types of power transistors mounted on the heat sink. I tested the larger (last stage) transistors (2SC3281 and 2SA1302), and they tested ok.

    The smaller power transistors (2SC3298A and 2SA1306A) tested ok, as well; however, when I was removing them for out-of-circuit testing, I noticed something important.

    All of the power transistors mount to the heat sink with the customary screw mount. When I was gently loosening one of the pair of smaller power transistors, I noticed that the solder joints were all loose! (Although I had tested continuity between solder joint and leads earlier, perhaps being in-circuit, or an intermittent connection gave me a false positive.)

    The looseness of the solder joints perhaps was due to a manufacturing defect, in which the leads of the smaller power transistor (2SC3298A and 2SA1306A) are bent to 90 degrees, in order to go down into the circuit board. If the leads are soldered to the circuit board before the screw is tightened, and also if the leads have too much spring tension in them, then, over a period of time, the spring action of those leads can break the solder joints, especially when high vibration is encountered. In the case of this amp, all 3 leads were loose.

    After removing both smaller power transistors (2SC3298A and 2SA1306A), I cleaned the old thermal grease from their footprints.
    Before I applied thermal grease or soldered, I did a 'trial run' by first bending the leads to an accurate 90 degree angle, ensuring no 'spring' or 'bend' in the leads once they were inserted into the circuit board mounting holes.

    Once the leads' bending angle was satisfactory, I reapplied thermal grease to each of the pair of smaller power transistors, and then screwed them to the heat sink, before soldering.

    Then (only after they were secure to the heat sink) I resoldered them to the board, making sure that the leads were bent to 90 degrees and with no spring action observed.

    This fixed the distortion encountered in the amp.

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    • #17
      So.
      My M-80 has been sitting around the room doing nothing since 2011, until i saw the last post. It appears that at the time i re-did every solder joint from the preamp board, but i did nothing to the power amp board.
      It was actually a loose joint on the CS3298B (mid-sized black left transistor on my 1992 unit). That would explain why prior to the definitive failure, i had intermittent cuts that could be resolved by smacking the top of the amp with my hand...
      I'm so happy to have this big boy running again !
      Thanks buck1107 !

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